1. Field of the Invention
The present invention relates to a method for gumming a lithographic printing plate. In particular, the present invention relates to a method wherein a hydrophilic protective layer is jetted on the lithographic printing plate.
2. Description of the Related Art
In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper.
Different technologies can be used to manufacture lithographic printing plates, e.g., conventional pre-sensitized lithographic printing plates bearing a UV sensitive coating, thermal plates that are sensitive to heat or infrared light, and more recently lithographic printing plates manufactured using inkjet printing technology.
In the final process of making a lithographic printing plate, “gumming up” is conducted in which a finisher (so-called “gum solution”) is applied onto the surface of the lithographic printing plate in order to protect the non-image (i.e., non-printing) areas thereof. The gum solution is typically applied to the printing plate by immersion of the printing plate in a bath of gum solution, e.g., in an automatic developing machine, or by spraying or jetting the gum as disclosed in EP 1524113 A (AGFA).
Such a gumming up step is performed for various purposes, for instance: for enhancing the hydrophilicity of the non-printing areas of the lithographic printing plate; for preventing the deterioration of the lithographic printing plate during the storage of the plate prior to printing operations or during the interruption period of the printing operations; for preventing the contamination of the lithographic printing plate with grease from fingers or hands or ink during handling the plate, which is often observed, for instance, when the plate is mounted to a printing press and hence for preventing the non-printing areas of the plate from being made ink receptive; and for preventing possible defects from appearing on the printing or non-printing areas thereof during handling the same.
A special type of gum solution, called baking gum, is used for protecting the plate during the baking step. A baking step involves heating of the plate so as to increase the run length during printing.
Once mounted on a printing press, the protective layer formed by the gum solution is removed from the lithographic printing plate by the fountain solution or the printing ink. It can be difficult, during printing, to adhere ink to the printing areas to which a thick layer of the gum solution is applied. For this reason, it takes a long period of time to obtain printed matters having a desired ink density. On the other hand, it is observed that the hydrophilicity is lowered in the non-printing areas to which a thin layer of the gum solution is applied. In this case, the non-printing areas are easily contaminated and hence cause background contamination of printed matters.